Refrigeration systems are most commonly employed for the purpose of cooling habitable environments for human comfort, a function called air-conditioning, and for cooling fluids or products to lower temperatures for commercial processes employed in manufacturing and in the processing and preservation of foods. All refrigeration systems operate between a lower temperature at which a heat collector must be maintained to produce the cooling effect desired, and a higher temperature at which a heat dissipator must be able to reject the sum of the heat picked up at the heat collector and the energy required to be put into the system to move the heat picked up from the lower to the higher temperature.
In vapor compression refrigeration practice the heat collector is called an evaporator because refrigerant liquid is evaporated to a vapor in it. This evaporation provides a cooling effect to material, fluid or solid, which is arranged in heat transfer relation to the evaporator. The heat dissipator is called a condenser because in it refrigerant vapor produced by the evaporator is condensed to a liquid for recycling back to the evaporator. The condensation within the condenser is achieved by arranging for a coolant to be placed or passed in heat exchange relation to the condenser. A motor driven compressor removes the vapor from the evaporator at a lower pressure and lower temperature and pumps it to the condenser at higher pressure where its heat can be dissipated to the coolant at a higher temperature.
In 1824 a French engineer, Nicholas Leonard Sadi Carnot, first published, in a paper titled "Reflections on the Motive Power of Fire, and on Machines Fitted to Develop that Power", a theory which developed the principle that a heat engine, (a vapor compression refrigeration system being one type of a heat engine), must have both a heat source and a heat sink. He further demonstrated that the efficiency of a heat engine is dependent on the temperature difference between the heat source and the heat sink. In refrigeration terms this means that for a condensing coolant of a given temperature the efficiency and capacity of a given refrigerating machine will decrease as the temperature desired at the evaporator decreases. Conversely, the efficiency of the refrigerating machine will increase as the temperature of the condensing coolant decreases.
In an effort to minimize the effect of Carnot's thermodynamic principle, and to improved the mechanical performance of mechanical gas compressors both of which are severely degraded with increasing pressure difference, various stratagems have been devised. Among these is the system where refrigerating compressors are placed in series so that each compressor has to pump over a smaller pressure difference than a single compressor performing the same function. This series compression is called a compound compression system. Another is the so-called cascade system, on which the present invention is an improvement. Cascade systems employ two or more separate vapor compression type refrigeration systems in series. The systems may each have the same type volatile refrigerant or may each have different types. In cascade systems a first system directly cools the fluid or product to be cooled. This first system, generally known as a low-temperature stage or `low-stage` system, has its heat rejecting element or condenser cooled by the refrigerating effect of a second refrigerating system which is commonly called the high temperature stage or high stage. A heat transfer liquid circulated by a pump between the cooling effect of the high-stage and the heat dissipating effect of the lowstage achieves the necessary heat transfer between the high-stage and the low-stage.
The present invention improves on the known cascade systems by providing an air-cooled heat-exchanger in the discharge line between the low-stage compressor and the low-stage condenser. The heat exchanger is positioned to be subject to an alternating cooler and warmer ambient. When the heat exchanger is subject to the warmer ambient, the usual high stage system must be operated to provide cooling for the low-stage condenser. However, when a cooler ambient is present around the heat exchanger, the high stage system is shut off and the required cooling for the low stage condenser provided by the air-cooled heat exchanger.
Control means for establishing the desirable operating modes of the various components, under various operating conditions, are described.
In another embodiment of the present invention, a refrigeration system is equipped with an evaporator for cooling a first fluid stream, such as air or liquid. A secondary heat exchange element conveying a second fluid to be cooled is positioned in heat transfer relation to the first fluid stream enroute to the evaporator. The second fluid, cooled by the secondary heat exchange element, is employed in either of two different ways: In one embodiment of the present invention the second fluid is employed to provide cooling to a process or system external to the parent system in whose fluid stream the second fluid is cooled. In that embodiment the refrigeration system having and cooling the secondary heat exchange element is the high stage of a cascade refrigeration system and the second fluid is routed to the low-stage condenser to provide cooling for it.
In another embodiment of the present invention the refrigeration system having and cooling the secondary heat exchange element is the low-stage of a cascade refrigeration system and the second fluid is the liquid refrigerant flowing from the low-stage condenser to the low-stage expansion device.